"When we dove down in the submarine, we noticed the water became murkier as we got closer to the bottom. ... We just saw this cloud but had no idea what was causing it," said Jesús Pineda, lead author of the study.

They thought they were looking at sedimentary rocks at first — that is, until the "rocks" started moving.

The researchers had discovered a giant cast of red crabs walking along the seafloor.

It came as a surprise for the group, because the red crab species had never been seen so far south: "To find a species at the extreme of their range and to be so abundant is very unusual," Pineda said.

The researchers say the discovery shows just how unpredictable and mysterious seamounts — undersea mountains — can be.